A motor is a device which converts electrical energy to work (rotational energy). As a typical motor, there is a synchronous motor, in which a rotor is attracted to a stator by a rotating magnetic field which is generated by an alternating current which is applied to the stator, thereby rotating to follow the rotating magnetic field. In particular, a permanent magnet synchronous motor (a PM synchronous motor) has high efficiency, compared to an induction motor, an electromagnet synchronous motor, or the like.
In a motor which is used in industrial machinery or the like such as an injection molding machine, it is required to generate high torque. As the permanent magnet synchronous motor suitable for such a purpose, an interior permanent magnet synchronous motor (an IPM synchronous motor) is attracting attention. This is a motor in which a permanent magnet is not stuck to the surface of a rotor but is embedded in the rotor.
A torque formula of a permanent magnet motor is given by Expression (1).T=p(Ld−Lq)IdIq+pϕ0Iq  (1)
p: number of poles of a motor
Ld, Lq: d-axis or q-axis inductance
Id, Iq: d-axis or q-axis current
ϕ0: magnet magnetic flux
The first term of Expression (1) is reluctance torque which is generated by a difference in magnetoresistance, and the second term is magnet torque which is generated by a magnetic force of a magnet.
The magnet torque is torque which is generated by attraction and repulsion between a permanent magnet and a rotating magnetic field, and is generated by a permanent magnet embedded in a motor.
Further, the reluctance torque is torque which is generated by saliency in which magnetoresistance changes depending on the position of a rotor. An axis along a magnetic field of the rotor is set to be a d-axis and an axis electromagnetically orthogonal to the d-axis is set to be a q-axis.
In the IPM synchronous motor, the flow of a magnetic flux in a d-axis direction passes through a permanent magnet having small magnetic permeability, and therefore, electromagnetic resistance becomes greater, and thus the d-axis inductance Ld becomes greater. On the other hand, a q-axis direction is directed to a direction of a side surface portion of a permanent magnet, and therefore, the flow of a magnetic flux does not pass through the permanent magnet and passes through a magnetic body having large magnetic permeability. Therefore, electromagnetic resistance becomes smaller, and thus the q-axis inductance Lq becomes smaller. The reluctance torque is generated by a rotating magnetic field, based on the difference between these inductances. As is apparent from Expression (1), the greater the difference between the inductances, the greater the reluctance torque becomes.